1. Residential Foundations
He who has not first laid his foundations may be able with great ability to lay them afterwards, but they will be laid with trouble to the architect and danger to the building. Niccolo Machiavelli ( ), The Prince
Civil Engineering and Architecture
© 2010 Project Lead The Way, Inc.

2. Residential Foundations
Civil Engineering and Architecture
Unit 2- Lesson 2.3 – Residential Design
Foundations
Purpose
Considerations
Types of Foundations
Shallow Foundations
Spread Footings
Strip Foundations
Slab-on-Grade and Thickened Slabs
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Copyright 2010

3. Purpose of Foundations
Residential Foundations
Civil Engineering and Architecture
Unit 2- Lesson 2.3 – Residential Design
Purpose of Foundations
Provide a level, stable surface to safely support a building
Transfer building loads to soil
Anchor the building from wind, flood, and seismic loads
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Copyright 2010

4. Design Considerations
Residential Foundations
Civil Engineering and Architecture
Unit 2- Lesson 2.3 – Residential Design
Design Considerations
Loads from the structure
Allowable soil bearing pressure
Frost depth
Flood elevation
Drainage
Costs
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Copyright 2010

5. Loads from the Structure
Residential Foundations
Civil Engineering and Architecture
Unit 2- Lesson 2.3 – Residential Design
Loads from the Structure
Foundations Must Resist
Dead Load
Weight of building
Live Load
Weight of occupants, furniture, and equipment
Lateral Loads
Wind
Seismic activity
Flood
The foundation must resist the design load combinations that are specified by building codes. For instance, dead and live loads must be considered together. Dead and live loads will be transferred through the structural components to the earth, which will cause a soil reaction pressure.
Some load combinations may actually cause the building to lift up from the foundation. For example, a load combination included in the IBC requires that only 60% of the dead load be combined with wind and lateral earth pressure. The wind and lateral earth pressure can cause a tipping effect on the building which will increase the soil reaction on one side of the building, but may actually lift up one side of the building. The foundation must be able to safely transfer both downward and upward forces into the earth.
SOIL REACTIONS
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Copyright 2010

6. Allowable Soil Bearing Pressure
Residential Foundations
Civil Engineering and Architecture
Unit 2- Lesson 2.3 – Residential Design
Allowable Soil Bearing Pressure
Indicates the maximum pressure that a soil may be designed to support
Typically presented in pounds per square foot (psf)
Different types of soils have different allowable soil bearing pressures
Project Lead The Way, Inc
Copyright 2010

7. Residential Foundations
Civil Engineering and Architecture
Unit 2- Lesson 2.3 – Residential Design
Soil Information
Local building department, codes, and regulations
Preliminary information: USDA Web Soil Survey
Local or state building codes
Soil testing/analysis
Site inspection and simple soil testing
Soil borings taken at proposed foundation locations
Information on types of soils present on a site and/or the allowable soil bearing pressure are available through many sources. However, since residential structures are relatively light in weight, an investigation of the allowable soil bearing pressure is rarely justified. Standard foundation types and sizes are based on local practice and minimum code requirements unless there are extenuating circumstances.
We will investigate soil bearing capacity later in the course when we design larger commercial facilities.
Project Lead The Way, Inc
Copyright 2010

8. Residential Foundations
Civil Engineering and Architecture
Unit 2- Lesson 2.3 – Residential Design
Frost Depth
Freezing of soil can cause heaving of foundations
Silt or clay soils with a high water table are highly susceptible to frost
Defense
Build base of foundation below frost depth
Provide frost protection for foundation
Project Lead The Way, Inc
Copyright 2010

9. Residential Foundations
Civil Engineering and Architecture
Unit 2- Lesson 2.3 – Residential Design
Frost Heave
This image shows the result of frost heave on an addition to a building. The new foundation was placed above the frost line. When the supporting wet soil froze, the ice build-up forced the foundation to lift up, causing cracking in the walls and floor.
Project Lead The Way, Inc
Copyright 2010

10. Frost Depth Contour Map
Residential Foundations
Civil Engineering and Architecture
Unit 2- Lesson 2.3 – Residential Design
Frost Depth Contour Map
This frost depth contour map shows approximate frost depths across the continental United States; however, it provides only an estimate. The numbers under each state symbol represent the average frost depth for the state. The contour lines give a better estimate. Find your location and make a note of the frost depth.
Note: the Affordable Home site is located approximately where the IN is placed on the map, and the frost depth is estimated at 20 inches.
The bottom of the foundation must be placed below this frost depth to prevent frost heave.
Be aware that this map provides only an approximation of the frost depth – local conditions will control. Check your local codes for frost penetration requirements.
Project Lead The Way, Inc
Copyright 2010

11. Residential Foundations
Civil Engineering and Architecture
Unit 2- Lesson 2.3 – Residential Design
Flood Elevation
Inundation by flood waters should be avoided
Damage to structure
Damage to contents
Height of floors is dictated by building codes and should be above flood levels
Courtesy Federal Emergency Management Agency. Photographer Dave Saville.
Project Lead The Way, Inc
Copyright 2010

12. Flood Insurance Rate Map (FIRM)
Residential Foundations
Flood Insurance Rate Map (FIRM)
Civil Engineering and Architecture
Unit 2- Lesson 2.3 – Residential Design
Base Flood Elevation
Affordable Home Site
FEMA produces Flood Insurance Rate Maps which locate special flood hazard areas. Special flood hazard areas are predicted to flood during a 100-year storm (i.e., areas that have a 1-percent chance of flooding in any given year). Base flood elevations (BFE) are given on the FIRM for flood-prone areas. A BFE is the predicted elevation of flood water during a flood and is used to design structures in these areas.
The Affordable Home site is not within a special flood hazard area.
FIRM area available on the FEMA online Map Service at
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Copyright 2010

13. Residential Foundations
Civil Engineering and Architecture
Unit 2- Lesson 2.3 – Residential Design
Drainage
Ground should be sloped away from the building
Provide drainage pipe along continuous foundations
Ground floor should be located 6" – 8" above grade
Your project design should promote water drainage away from the structure.
Project Lead The Way, Inc
Copyright 2010

14. Residential Foundations
Civil Engineering and Architecture
Unit 2- Lesson 2.3 – Residential Design
Drainage
8"
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Copyright 2010

15. Residential Foundations
Civil Engineering and Architecture
Unit 2- Lesson 2.3 – Residential Design
Shallow Foundations
Transfers loads to the soil very near the surface
Spread footing or strip footing
Slab-on-grade
Most residential foundations are shallow foundations (i.e., the foundations bear on soil very close to the surface).
Project Lead The Way, Inc
Copyright 2010

16. Spread (Column) Footing
Residential Foundations
Civil Engineering and Architecture
Unit 2- Lesson 2.3 – Residential Design
Spread (Column) Footing
COLUMN
LOAD
A footing that spreads the load over a broad area which supports one (or a few) load(s)
USES
Under piers or columns
PIER (Concrete or Masonry)
SPREAD FOOTING (Concrete)
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Copyright 2010

17. Continuous (Strip) Foundation
Residential Foundations
Civil Engineering and Architecture
Unit 2- Lesson 2.3 – Residential Design
Continuous (Strip) Foundation
A wide strip of reinforced concrete that supports loads from a bearing wall
USES
Under foundation walls
For crawl space/basement
LOAD
FOUNDATION WALL (Concrete or Masonry)
STRIP FOOTING
(Concrete)
Project Lead The Way, Inc
Copyright 2010

18. Slab-on-Grade and Thickened Slab
Residential Foundations
Civil Engineering and Architecture
Unit 2- Lesson 2.3 – Residential Design
Slab-on-Grade and Thickened Slab
THICKENED SLAB
WALL
SLAB-ON-GRADE
Slab-on-Grade – Reinforced concrete floor supported by soil
Thickened Slab – A slab on grade with an integral footing created by thickening the slab
USES
Shallow frost depth or when frost protection is used (instead of strip footing)
Project Lead The Way, Inc
Copyright 2010

19. Residential Foundations
Civil Engineering and Architecture
Unit 2- Lesson 2.3 – Residential Design
Residential Foundations
Purpose
Considerations
Types of Foundations
Shallow Foundations
Spread Footings
Strip Foundations
Slab-on-Grade
Thickened Slabs
Project Lead The Way, Inc
Copyright 2010